Acinetobacter species are ubiquitous in nature (Baumann, 1968; Juni, 1972). Acinetobacter is commonly found in soil, water, food and also on healthy human skin. (Saha & Chopade, 2001; Patil & Chopade 2001; Dhakephalkar et al, 1994a). Acinetobacter is one of the known opportunistic human pathogens (Dhakephalkar & Chopade 1994a; Chopade et al, 1994a & 1994b). It also possesses a number of naturally occurring plasmids exhibiting resistance to antibiotics and heavy metals (Deshpande et al 1994; Shakibaie et al, 1999). There are few reports on the presence of Acinetobacter in soil; however, detailed studies regarding its occurrence, distribution, growth pattern, physiology and interactions with other soil microorganisms are not known. There is only one statement on the presence of Acinetobacter in wheat rhizosphere (Kleeberger et al, 1983). However, detailed information about the role of Acinetobacter in rhizosphere is not known.
Soil is a rich environment for growth of microorganisms, and specifically rhizosphere is a highly specialized environment in soil for growth of microorganisms. Since rhizosphere contains a large number of microorganisms one would expect plasmid transfer and dynamics of plasmid transfer from Acinetobacter to other microorganisms and vice versa in the rhizosphere environment. The rhizosphere of each and every plant is very specific with respect to the root exudates, as they are the main source of nutrients for rhizosphere microorganisms (Subba Rao, 1986).
Until now there has been no report on the involvement of plasmids in the production of IAA from the genus Acinetobacter. IAA is one of the major plant growth promoting hormones produced by plants as well as some bacteria and fungi. (Arshad & Frankenberger, 1991). Many species of bacteria produce IAA, especially when growth media are supplemented with tryptophan, a precursor of IAA. A number of microorganisms like Agrobacterium tumefaciens, Agrobacterium rhizogenes, Pseudomonas savastanoi, Pseudomonas spp., (Leinhos & Vocek, 1984), Rhizobium spp, (Baldi et al, 1991), Bradyrhizobium spp. and Azospirillum spp (Bashan et al, 1989) present in the rhizosphere of plants are known to produce IAA (Costacurta & Vanderleyden, 1995). To some extent, the biosynthesis of plant growth promoting substances such as auxins from phosphate solubilizing rhizobacteria from rhizosphere of wheat and rye has been reported (Leinhos & Vocek, 1984).
The aim of the present work was to isolate and characterize Acinetobacter from rhizosphere of wheat and to find out the role of Acinetobacter in plant growth promotion in general and involvement of plasmids in the production of plant growth promoting substance such as indole acetic acid (IAA).
Acinetobacters were isolated from rhizosphere of wheat. The variety of wheat plant was HD 2189 ICAR, New Delhi, India. The rhizosphere soil was collected throughout the December to March 1998 growing season, at different stages of the life cycle of the wheat plant: control soil (0 d), elongation (30 d) flowering stage (45 d), fruiting stage (60 d) and ripened fruiting stage (75 d). The rhizosphere was collected from three areas—rhizosphere soil (RS), rhizoplane (RP), and non rhizosphere soil (NRS). The samples were collected from an agricultural field of Mahatma Phule Agriculture College, Shivajinagar, Pune, Maharashtra, India. Samples were brought to a laboratory and processed immediately within half an hour.